hydraulic braking device having security provisions, for a farm tractor or a similar vehicle

ABSTRACT

A braking system for farm tractors and similar vehicles, has a differential braking system and a hydraulic system for the braking assistance forming a system unit (H) which includes two circuit branches (H 1 ,H 2 ), each circuit branch operating the braking with respect to one side of the vehicle. To the system unit are applied, through connection interfaces ( 36,37 ), two control units (C 1 ,C 2 ), each control unit including an electrovalve ( 20 ) and a distributor ( 24 ) operated by the electrovalve, each distributor controlling the operation of the system unit (H) with respect to one of the circuit branches, and each electrovalve being singularly controlled by an electronic control circuit (EL) provided with sensors of the main vehicle parameters, such as a sensor (V) of the vehicle speed, sensors (R) of the speed of singular wheels, sensors (P) of the positions of the braking pedals and a sensor (A) of the position of the accelerator pedal.

The subject of this invention is a hydraulic braking device, mainlyintended for a farm tractor or a similar vehicle, which is characterizedby the presence of special provisions destined to provide securityfunctions.

In some vehicles, particularly the farm tractors and like vehicles, thehydraulic braking system includes two master cylinders operating aspumps, which are controlled by two separate braking pedals. Each ofthese master cylinders operates the braking of one of the rear sidewheels (left-hand and right-hand wheels) of the vehicle, which are thedriving wheels. This arrangement has the purpose of allowing aunilateral braking of the side wheels, for example in order to help thevehicle rotation by the U-turns when a field is being worked alongadjacent lanes which are run in alternatively opposite directions. When,on the contrary, a bilateral braking is commanded by operating both thebraking pedals, in general the system could be unbalanced. because thetwo separate braking circuits take up different oil quantities, and theresult would be a similarly unbalanced braking. In order to prevent thisdrawback, a balancement channel is provided that mutually communicatesthe two braking circuits when both the braking pedals are operatedsimultaneously. The balancing function is effected by balancing valvesinstalled on the pistons of the two master cylinders, these valves beingnormally closed and being opened after the pistons have covered apreestablished stroke.

Each master cylinder of this braking system is a part of a brakingdevice which may comprise arrangements for the braking assistance. Forthe purposes of this invention it will be mainly taken in considerationa hydraulic device improved to the aim of increasing the brakingsensitivity, such as it has been described in detail in the EuropeanPatent No. 1.457.400. This hydraulic braking device comprises a mastercylinder and a hydraulic servosystem for braking assistance, including abody wherein there is sealingly and slidably mounted a hydraulicassistance piston which mutually separates a first chamber and a secondchamber, these chambers being put in mutual communication and beingfilled with a hydraulic fluid under high pressure at rest, whereas atthe time of a braking operation the communication between these twochambers is interrupted and a communication between the second chamberand the master cylinder is opened, thus causing the displacement of theassistance piston.

Another security arrangement used in a braking system of this kind is afunction which, in the case of a default of the high pressure oilcircuit, ensures by the muscular force of the user a braking that cannotbe assisted but is usually sufficient for an emergency braking.

The modern farm tractors may be intended to attain high speeds, up to 80km/h and over, when they are running on road, and therefore there areneeded some active and passive security provisions that are known forthe road vehicles but, in the past, were not considered needed for thesevehicles. For example, some regulations impose that, for the sake ofsecurity, the tractors attaining speeds over 40 km/h be provided with abraking device with double circuit.

Other security provisions used in the road vehicles comprise, forexample, a function of regulation of the slip in traction(ASR=Acceleration Slip Regulation), a function preventing the wheelblockage in braking (ABS=Anti-lock Braking System), and a function ofstability control (ESP=Electronic Stability Program). These securityfunctions, or some of them, could be very useful for the farm vehiclestoo, however they have not yet been applied in the farm tractors andsimilar vehicles, mainly because their adoption should have encounteredheavy difficulties in view of the special braking system present inthese vehicles.

With reference to the braking systems for farm tractors and similarvehicles that are equipped with a differential braking system and alsowith a hydraulic system for braking assistance (technically namedhydrobooster), the main object of this invention is to introduce in saidsystems at least one additional security function, such as a function ofregulation of the slip in traction (ASR), or a function preventing thewheel blockage in braking (ABS), or a function of stability control(ESP), or even other functions, and preferably several of thesefunctions. As it may be understood, the introduction of these functionsensures a further important security for the vehicles of the consideredkind, when they are suitable for attaining high speeds on road.

Another object of the invention is to ensure a correct system operationin the different operating condition that may occur. Still anotherobject of the invention is to attain the stated objects by reducing at aminimum the complication and the encumbrance of the system, withparticular reference to its installation onto the vehicle. Finally, itis an object of this invention to limit at a minimum the expensesinvolved in the provision of this improved braking system.

In a braking system for farm tractors and similar vehicles whichcomprise a differential braking system and a hydraulic system for thebraking assistance forming a system unit which includes two circuitbranches, each circuit branch operating the braking with respect of oneside of the vehicle, the main invention object is attained because tosaid system unit having two branches are applied, through connectioninterfaces, two control units, each control unit including anelectrovalve and a distributor operated by said electrovalve, each saiddistributor controlling the operation of the system unit with respect toone of the circuit branches, and each said electrovalve being singularlycontrolled by an electronic control circuit provided with sensors of themain vehicle parameters.

The control electronic circuit may be of a substantially known type andmay be operated by a control logic of the so-called “ECU” type, whichtakes into account the main vehicle parameters observed by means of perse known sensors, such as sensors of the operation of the brakingpedals, a sensor of the vehicle speed, sensors of the rotationalvelocity of singular wheels, acceleration sensors and other possiblesensors, and is suitable for ensuring a correct braking in any runcondition, whereby the vehicle can be dominated in any condition.

This invention finds its preferred application in those braking systemsin which the assistance to braking is provided by a source of highpressure fluid, and in such applications the pressure used for operatingthe one or the other of the additional security functions, or severalsuch functions, is the same pressure provided by the high pressuresource supplying the assistance system.

Preferably the electrovalve of each control unit is a three-positionselectrovalve, a first position being a rest position, a second positionbeing a position of operation of the ASR function, and the thirdposition being a position of operation of the ABS function.

It is of advantage that said electrovalve has its first position locatedin center and the second and third positions displaced in oppositedirections with respect to said central first position.

Preferably, the distributor of each control unit has a series of gatesthat are mutually connected in various manners by a distribution slideoperated by the respective electrovalve, each said port being connectedto a chamber or passageway of the respective hydraulic device of thesystem unit.

Preferably, the distributor of each control unit, when it is in the restposition, does not modify in any manner the connections of the chambersof the respective hydraulic device.

On the contrary, the distributor of each control unit, when it is in theposition for activating the ASR function, modifies the connections ofthe chambers of the respective hydraulic device, by isolating from thesystem the discharge passageway and commutating the connection of thechamber connected to the wheel having the lower adherence from thedischarge position to a pressure position.

And the distributor of each control unit, when it is in the position foractivating the ABS function, modifies the connections of the chambers ofthe respective hydraulic device, by commutating the connection of theintermediate chamber from the condition of communication with thedischarge passageway to a condition of communication with the highpressure supply passageway, and connecting the assistance chamber to thedischarge passageway, in order to generate an antagonist force aiming toreduce, even to the annulment, the assistance effect and, if needed,even to counteract against the muscular force exerted by the driver.

Each master cylinder of the system unit comprises a device suitable forintercepting the balancement channel extending between the two mastercylinders, when a pressure is sent to a chamber of a master cylinder inorder to activate an ASR or ABS function, whereby it is then allowed toapply different pressures to the braking circuits of the two vehiclesides.

It is of advantage that said interception device includes a shutter witha spring returned piston which is subjected through a pipe to thepressure present in the intermediate chamber of the master cylinder, andintercepts the balancement channel by displacing against the springaction.

The braking device of the invention may operate the braking action onlyonto the rear wheels of the vehicle, but it may also be arranged foroperating the braking of the front wheels or the wheels of a trailertoo. In such a case it may be equipped with a disconnection valve whichshuts out the braking of the front vehicle wheels or the trailer wheelswhen a unilateral braking is effected.

These and other features, objects and advantages of the subject of thisinvention will appear more clearly from the following description of anembodiment, which has the character of a non-limiting example, whereinboth the ASR and the ABS functions are foreseen. It is however intendedthat the invention may be embodied by providing only one of thesefunctions or even other functions. With reference to the appendeddrawings:

FIG. 1 shows a perspective view of a braking device according to theinvention and the block diagram of an electronic control circuitconnected to the braking device.

FIG. 2 shows a longitudinal section of one of the master cylindersoperating as pumps, forming a branch of the two-branches system unit ofthe braking system.

FIG. 3 shows a diagrammatic section of one of the two control unitsapplied to the system unit, comprising an electrovalve and adistributor, represented in the rest condition.

FIG. 4 shows the component parts represented in FIG. 3, but representedin the condition provided for intervention of the function of regulationof the slip in traction (ASR).

FIG. 5 shows the component parts represented in FIG. 3, but representedin the condition provided for intervention of the function of preventionof the wheel blockage in braking (ABS).

FIG. 6 shows a view of a control unit having an interface forcommunication with the system unit.

FIG. 7 shows in perspective the body of the system unit, having aninterface for communication with a control unit.

FIGS. 8 to 10 refer to an arrangement of the master cylinders forinterrupting the balancement action when takes place the intervention ofone of the additional functions of regulation of the slip in traction(ASR) and/or of prevention of the wheel blockage in braking (ABS).

With reference to FIG. 1, there is represented in perspective thebraking device according to the invention, which comprises a centralunit H (so-called hydrobooster) formed by the coupling of two hydraulicdevices H1 and H2, each substantially corresponding to the hydraulicdevice described in the European Patent No. 1.457.400. This device issummarily described here, and for the details of its structure andoperation reference is made to the cited European Patent No. 1.457.400.

FIG. 2 shows a section of one of the two hydraulic braking devices H1and H2. This device comprises a body 10 wherein there is made a bore 11having two diameters, which in the rear portion forms a chamber 6 oflarger diameter forming a hydraulic braking servomotor, and in the frontportion of reduced diameter forms a master cylinder operating as a pump,connected through a connection 7 to the brake of one of the vehiclewheels. In the bore 11 slides a control piston 12 driven by a commandshaft 13 that ends at a braking pedal (not shown). In chamber 6, aroundthe control piston 12 is slidingly mounted an assistance piston 14,behind which a chamber 5 is normally connected by a passageway 4 to ahigh hydraulic pressure source (not shown) intended to supply theassistance. Further two chambers, a chamber 2 inside the bore 11 and anintermediate chamber 3, are separated the one from the other by anannular piston 8 and, at rest, they are connected to a dischargepassageway 1. The intermediate chamber 3 is separated from chamber 6 bya movable member 15. At rest, in chamber 6 is present the same pressurethat is present in chamber 5 but, when the control piston 12 isoperated, it controls openings which modulate the pressure in chamber 6,thus causing the intervention of the assistance, as it is described indetail in the European Patent No. 1.457.400.

According to the invention, as already stated, and as it appears fromFIG. 1, to the system unit H having two branches are applied two controlunits C1 and C2, and each of them, under the command of an electroniccontrol circuit EL, modifies the behavior of the system unit H withreference to the braking of the wheels situated at one side of thevehicle, when it is required the intervention of a function ASR, ABS orother function effected through the braking control. In order toaccomplish its function, the electronic control circuit EL gets signalsfrom several sensors, such as a sensor V of the vehicle speed, sensors Rof the rotational velocity of the wheels, sensors P of the operation ofthe braking pedals and sensors A of acceleration. The electronic controlcircuit EL sends its control signals to the electric connections 29 ofthe electrovalves included within the control units C1 and C2.

FIG. 3 diagrammatically shows the longitudinal section of one of the twocontrol units C1 and C2, represented in the rest condition. This unitcomprises an electrovalve body 20 wherein are housed two magnetic cores21 and 22, which are mutually coupled and are operated by respectiveelectric windings (not shown) having electric connections 29. Themagnetic cores 21 and 22 are intended for being subjected todisplacements in the opposite directions with respect to a central restposition shown in FIG. 3. The group of magnetic cores 21 and 22 operatescorresponding displacements of a distribution slider 23 slidinglymounted within a distribution body 24. The slider 23 cooperates withfive ports 31, 32, 33, 34, 35 formed in the body 24, by modifying themutual connections thereof, as it will clarified later on. The fiveports 31, 32, 33, 34, 35 are connected, in this order, to the chambers1, 2, 3, 4, 5 of the corresponding hydraulic device (H1 or H2) of thesystem unit H. This connection is done through passageways provided inthe body of the apparatuses and ending at an interface 37 of thedistributor body 24 (FIG. 6) and at a corresponding interface 36 (FIG.7) of the two interfaces present at the two sides of the body 10 of thesystem unit with two branches. In said interfaces 36 and 37 are hollowedseveral channels connecting the outlets of the passageways. For the sakeof clarity these outlets are designed by the same reference numbers 1,2, 3, 4, 5 and 31, 32, 33, 34, 35 that designate the chambers of themaster cylinder and the ports of the distributor, from which come thecorresponding passageways. Therefore, port 31 is a discharge portcommunicating with the discharge passageway 1, port 32 communicates withthe inner chamber 2 of bore 11 of the hydraulic device, port 33communicates with the intermediate chamber 3, port 34 is incommunication, through passageway 4, with the supply of high pressurefor assistance, and port 35 communicates with the assistance chamber 5.Moreover, the intermediate chamber 3 is in communication with apassageway 30 which will be defined later on, serving for a function ofbalancement suppression.

In this rest condition, the control unit C1 or C2 does not modify thebehavior of the corresponding branch of the system unit 10, whichtherefore operates in the same way as if no control unit were present.This is the condition of normal run of the vehicle.

It will now be described the function of regulation of the slip intraction (ASR), whose addition to a hydrobooster is one of the objectsof the invention.

The farm tractors, when they are operating on field, can take advantageof the differential blockage that solves the adherence problems whenthey are encountered, but this is not possible on road. On the road,when a different adherence to the land arises for the driving wheels,the torque which can be transmitted is the lesser one, and some slipscan take place. In this case may be suitable an action of regulation ofthe slip in traction (ASR).

When the electronic control circuit EL ascertains, by the signalsreceived from the sensors, that is suitable to apply to a vehicle wheela function ASR for regulating the slip in traction, this electroniccontrol circuit EL activates the electric winding which cooperates withthe magnetic core 21 of the electrovalve of the control unit C1 or C2that corresponds to the side wherein is placed the wheel to becontrolled, and this magnetic core moves and drags the slider 23 bydisplacing the distributor from the rest position of FIG. 3 to theoperating position represented in FIG. 4.

In this condition, slider 23 cooperates with the ports 31 to 35 andisolates from the system the port 31 and therefore the dischargepassageway 1, whereas port 32 is commutated from port 31 to port 34 and,therefore, from the discharge condition to the high pressure condition.As a consequence, in chamber 2, which at rest is connected to chamber 28and to the braking circuit of the wheel having the lower adherence, isgenerated a pressure that aims to equilibrate with the pressures presentin chambers 4 and 5. In this way, even if the braking pedal remains inthe rest position, the wheel having the lower adherence is braked. Theamount of the braking torque is regulated by swift alternatecommutations of port 32 with ports 31 and 34, these commutations beingcaused by the electronic control circuit EL, and in this way the brakingtorque is modulated as long as persists a slip different between thedriving wheels.

When the electronic control circuit EL ascertains that it is suitable tocease the function of regulating the slip, it breaks off the activationof the electric winding of the electrovalve, and this latter reverts toits rest position, whereby slider 23 finally connects port 32 to thedischarge port 31, thus reestablishing a correct distribution of thedriving torque.

Usually, the assistance pressure is sufficient for compensating thedifferent driving torque between the driving wheels. The describedprocess ensures a progressive development of the braking regulation, towhich ensues a comfortable sensation for the vehicle user, who unlikelynotices the system intervention.

It will now described the function for preventing the wheel blockage inbraking, whose addition to a hydrobooster is one of the objects of theinvention.

The function for preventing the wheel blockage in braking (ABS) has thepurpose of automatically reducing the braking torque, by means of alesser liquid pressure in the involved braking circuit, to the one orthe other wheel, by work of the electronic control circuit EL, as aconsequence of the reception of a signal of the sensors that advises theimminent blockage of one or more wheels. The system is operated byelectronic way at the level of control, but it is completelyelectro-hydraulic in the operational matter.

To this purpose the circuit is carried out in such a manner that, whenit is activated, it reabsorbs a certain volume of braking liquid thanksto the presence, in the master cylinder operating as a pump, of amovable element 15 located within the inner chamber 6, which it dividesfrom the intermediate chamber 3. Such geometry allows, in certainoperating conditions, a return of braking liquid. In correspondence withthis liquid return the user feels the pedal to throb and notices abackwards push when the system reduces the braking force applied to awheel. However this effect is limited and it is considered as allowable.

When, during a braking operation, the electronic control circuit noticesthat it is suitable to activate on a wheel the antilock function, itactivates the electric winding cooperating with the magnetic core 22 ofthe electrovalve of the control unit that corresponds to the sidewherein is placed the wheel to be controlled, and this magnetic coremoves and drags the slider 23 by displacing the distributor from therest position of FIG. 3 to the operating position represented in FIG. 5.

In this condition, slider 23 cooperates with the ports 31 to 35 andcommutates the connection of port 33 from the condition of communicationwhich the discharge port 31 to the condition of communication with port34 which is under high pressure, whereas the discharge port 31 isconnected to port 35.

Therefore in the master cylinder, in chamber 3, the pressure increase,due to the admittance of a high pressure, generates a force antagonistto the force of chamber 6, wherein is present a pressure proportional tothe muscular force applied to the command shaft 13. This pressure aimsto equilibrate with the pressure present in Chamber 5, up to theannulment of the assistance effect and even, when needed, to counteractagainst the muscular force exerted by the driver.

The amount of the braking torque applied to the controlled wheel isregulated by the electronic control circuit EL through swift alternatecommutations of port 33 with ports 31 and 34, with the consequentexchange of ports 31 and 35, thus causing the reduction of theassistance pressure and, as a consequence, the reduction of the brakingpressure.

Further arrangements similar to those described can be foreseen by thedesigner in order to execute other security functions, under control ofthe electronic control circuit EL. In general, the idea of the inventionmay be applied for introducing any function that is controlled by theelectronic control circuit EL and is actuated by acting onto the brakingsystem.

In order that in particular conditions different pressure may be appliedto the braking systems of the wheels located at the opposite sides ofthe vehicle, it is needed to neutralize the balancement which, in thenormal conditions, ensures that said braking systems receive identicalpressures. The arrangements intended to this purpose are shown in FIGS.8 to 10.

FIG. 8 represents a cross section of body 10, wherein are formed theworking chambers 28 of the two master cylinders, and FIG. 9 is avertical section thereof. The working chambers 28 are mutually connectedby a balancement channel 27 which, in a manner per se known,communicates both master cylinders in order to balance their pressureswhen both braking pedals are operated. In body 10 of each mastercylinder is housed a piston shutter 25 put under the action of a spring26, which shutter is subjected, through a passageway 30, to the pressurepresent in the intermediate chamber 3. The piston shutter 25 isrepresented in detail, on a larger scale, in FIG. 10. The piston shutter25 is susceptible, by displacing against the action of spring 26, tointercept the balancement channel 27. Therefore, as long as persists apressure in chamber 3, the piston shutter 25 which is biased by thispressure intercepts the balancement channel 27 and inhibits itsfunction. This allows, when needed, to apply different pressures to thebraking circuits of the two vehicle sides, as it is needed in order ofactuating the ABS function, the ASR function or even other functions.

As already stated, the braking device according to this invention caneffect the braking onto the rear vehicle wheels only, but it may also bedesigned for operating the braking onto the front vehicle wheels or ontothe wheels of a trailer too. In this case the device can be equippedwith a disconnection valve intended to exclude the braking of the frontwheels or of the trailer wheels when a unilateral braking is effected.The fitting 9 shown in FIGS. 1 and 7 is intended to the connection ofthe braking circuit of the front wheels or of the trailer wheels when adisconnection valve is present.

As it appears from the foregoing, the application of the inventionbrings about to the farm tractors and similar vehicles the one or theother of the security functions ASR, ABS or possibly others, andpreferably more than one of these functions, thus making the vehiclesuitable to run on road even at high speed, and the invention attainsthis object by means of a compact device whose installation onto thevehicle does not involve difficulties and does not comprise particularlyexpensive equipments, such as special hydraulic centrals.

It is to be understood that the invention is not limited to theembodiment described and illustrated by way of example. Several possiblemodifications have been mentioned in the course of the description, andothers are within the reach of those skilled in the art. These and othermodifications and any replacement by technically equivalent means may beapplied to what has been described and illustrated, without departingfrom the spirit of the invention and the scope of this Patent.

1. A braking system for farm tractors and similar vehicles whichcomprise a differential braking system and a hydraulic system for thebraking assistance forming a system unit (H) which includes two circuitbranches (H1, H2), each circuit branch operating the braking withrespect to one side of the vehicle, characterized in that to said systemunit (H) having two branches (H1, H2) are applied, through connectioninterfaces (36,37), two control units (C1, C2), each control unitincluding an electrovalve (20) and a distributor (24) operated by saidelectrovalve, each said distributor (24) controlling the operation ofthe system unit (H) with respect to one of the circuit branches (H1,H2),and each said electrovalve (20) being singularly controlled by anelectronic control circuit (EL) provided with sensors (V, R, P, A) ofthe main vehicle parameters.
 2. A braking system as set forth in claim1, characterized in that said control electronic circuit (RL) isoperated by a control logic of the so-called “ECU” type, which takesinto account the main vehicle parameters, observed through sensors (P)of the operation of the braking pedals, a sensor (V) of the vehiclespeed, sensors (R) of the rotational velocity of singular wheels,acceleration sensors (A) and other possible sensors.
 3. A braking systemas set forth in claim 1, of the type in which the assistance to brakingis provided by a source of high pressure fluid, characterized in thatsaid pressure used for operating the one or the other of the additionalsecurity functions, or several such functions, is the same pressureprovided by the high pressure source supplying the assistance system. 4.A braking system as set forth in claim 3, characterized in that saidelectrovalve (20) of each control unit (C1,C2) is a three-positionselectrovalve, wherein a first position (FIG. 3) is a rest position, asecond position (FIG. 4) is a position of operation of the ASR function,and the third position (FIG. 5) is a position of operation of the ABSfunction.
 5. A braking system as set forth in claim 4, characterized inthat said electrovalve (20) has its first position located in center andthe second and third positions displaced in opposite directions withrespect to the central first position.
 6. A braking system as set forthin claim 1, characterized in that the distributor (24) of each controlunit (C1,C2) has a series of gates (31-35) that are mutually connectedin various manners by a distribution slide (23) operated by therespective electrovalve (20).
 7. A braking system as set forth in claim6, characterized in that the distributor (24) of each control unit(C1,C2) has a series of gates (31-35) that are mutually connected invarious manners by a distribution slide (23) operated by the respectiveelectrovalve (20), each said port (31.35) being connected to a chamberor passageway (1-5) of the respective hydraulic device (H1,H2) of thesystem unit (H).
 8. A braking system as set forth in claim 4,characterized in that the distributor (24) of each control unit (C1,C2),when it is in the rest position, does not modify in any manner theconnections of the chambers of the respective hydraulic device (H1,H2).9. A braking system as set forth in claim 4, characterized in that thedistributor (24) of each control unit (C1,C2), when it is in theposition for activating the ASR function, modifies the connections ofthe chambers of the respective hydraulic device (H1,H2), by isolatingfrom the system the discharge passageway (1) and commutating theconnection of the chamber (8) connected to the wheel having the loweradherence from the discharge position (1) to a pressure position (4).10. A braking system as set forth in claim 4, characterized in that thedistributor (24) of each control unit (C1,C2), when it is in theposition for activating the ABS function, modifies the connections ofthe chambers of the respective hydraulic device (H1,H2), by commutatingthe connection of the intermediate chamber (3) from the condition ofcommunication with the discharge passageway (1) to a condition ofcommunication with the high pressure supply passageway (4), andconnecting the assistance chamber (5) to the discharge passageway (1),in order to generate an antagonist force aiming to reduce, even up tothe annulment, the assistance effect and, if needed, to counteractagainst the muscular force exerted by the driver.
 11. A braking systemas set forth in claim 1, characterized in that each master cylinder (28)of a system unit (H1,H2) comprises a device (25-26) suitable forintercepting the balancement channel (27) extending between the twomaster cylinders, when a pressure is sent to a chamber of a mastercylinder in order to activate an ASR or ABS function, whereby it is thenallowed to apply different pressures to the braking circuits of the twovehicle sides.
 12. A braking system as set forth in claim 11,characterized in that said interception device (25-26) includes ashutter with a spring (26) returned piston (25) which is subjectedthrough a pipe (30) to the pressure present in the intermediate chamber(3) of the master cylinder, and intercepts the balancement channel (27)by displacing against the spring action.
 13. A braking system as setforth in claim 1, characterized in that, when the braking device (H) isarranged for operating the braking of the front vehicle wheels or thewheels of a trailer too, it is equipped with a disconnection valve whichshuts out the braking of the front wheels or the trailer wheels when aunilateral braking is effected.